Brakes on loom weft yarn feeding devices are suitable for maintaining a desirable tension profile in the weft yarn unwinding from the weft feeder drum, thereby preventing it from breaking. A weft yarn feeder is intended to feed weft yarn coming from a bobbin to a loom. Traditionally, a weft yarn feeder is provided with a braking member placed in the vicinity of the storing drum and acting in the direction of the drum to which the weft yarn is conveyed before exiting therefrom. The braking member has the function of keeping the weft yarn tension as constant as possible, and of preventing the yarn from breaking, which would inevitably cause at least a temporary interruption of the production cycle.
Known brakes for loom weft yarn feeding devices of the lever or ring type are built in such a way as to act systematically in the direction of the drum, and unfortunately possess a number of important disadvantages. The braking member, generally consisting of a disk acting counter to the weft yarn unwinding direction, tends to become deformed in time and then is no longer able to couple with the drum with the required precision. Moreover, during rotation of the withdrawn weft yarn, centrifugal forces tend to lift the weft yarn off the drum whereas the braking member tends to contrast it in the opposite direction. As a result, weft yarn tension, especially at higher withdrawal speeds and higher rotation speeds, increases considerably. The weft yarn itself thus becomes easily subject to breaking, or does not enable a reliable changeover between the loom pincers, e.g. in a rapier loom.
Another drawback with a known braking device using a brake cap generally produced from a plastic material is that the force exerted on the brake cap results in deformations of and wear spots at certain parts of the brake cap. Consequently, the yarn is no longer braked uniformly and exists from the weft yarn feeding device under braking that is not constant. This increases the danger of stretching and breaking the weft yarn.
The brake body of a brake as known from GB 14 25 900, FIG. 8, is an annular tube of circular cross-section made of rubber or plastic material. Said tube is inflated by pressurized air and is seated on a ring provided in the interior of the annular tube. Said ring is mounted to the drum front end portion of the storage drum such that a circumferential region of said annular tube is defining a circumferentially continuous braking rim which is radially held in contact with the covering element inner wall. In operation, supposedly due to the radially oriented force pressing the braking rim against the covering element inner wall such that the braking rim is acting cross-wise or even slightly opposite to the weft yarn withdrawal direction and/or due to the stiffness of the inflated tube, the weft yarn tension occurring in the weft yarn downstream the braking zone significantly varies with varying weft yarn speeds. In another embodiment (GB 1 425 900, FIG. 3), an annular brake body with axially oriented bristles is mounted in the space confined by the cone-shaped covering element. Tips of the bristles radially contact the covering element inner wall. Said bristles define an indefinite plurality of discrete braking tips, but no circumferentially continuous braking rim. This does not allow to achieve a relatively constant downstream tension in the weft yarn when the withdrawal speed of the weft yarn varies, since the friction between the braking tips of the bristles and the yarn progressively increases with increasing weft yarn withdrawal speed.
An object of this invention is to produce a loom weft yarn feeding device having a brake apt to guarantee optimal and constant tension of the weft yarn fed to the loom and to avoid the possibility of yarn breakages.
A further object of the invention is to produce a loom weft yarn feeding device having a brake in which the braking member provided for yarn braking is not subject to local deformation and/or wear which could prevent that the yarn passes the brake under constant braking conditions.
A further object of the invention is to put a brake in a loom weft yarn feeding device at the disposal of users, designed such that it is able to ensure high level resistance and dependability in time, while also being simple and economical to manufacture.
A further object of the invention is to create a loom weft yarn feeding device with a brake having an elastically deformable braking rim wherein the brake has an improved behavior in operation, leading to a relatively constant tension profile in the weft yarn, i.e. avoiding critical tension variations with varying weft yarn speed in weft yarn feeders for different types of looms, like projectile looms, rapier looms, and even jet looms. The brake ought to have an extremely simple design, should operate without changing its braking behavior due to wear and should be suitable for a broad variety of different yarn qualities. The operational behavior of the brake should lead to a self-compensating effect, which means that the brake automatically reduces the braking effect with increasing yarn speed or during temporary acceleration phases and vice versa.